Drive device and method for heating elements in a recording apparatus

Abstract: A drive device for selectively driving, through a plurality of recording cycles, a plurality of N heating elements (50) in the recording means, such as a thermal print head, of a recording apparatus, wherein the energizing period for each heating element in each cycle is divided into at least M successive subperiods (TW0, TW1; TW2, TW3, TW4), comprises first to M-th storage means (11, 12) for storing first to M-th sets of recording data each set including data for said plurality of heating elements, said first to M-th sets of recording data corresponding to a current cycle and the (M-1) preceding cycles, timing means (21-25, 31-34) for defining said at least M subperiods, first means (13-15) for generating, depending on the respective recording data in each of the sets of recording data corresponding to the current cycle and the first to (M-1 )-th preceding cycles, a signal for energizing a respective heating element during a first one of said subperiods, at least second means (17, 16) for generating, depending on the respective recording data in the set of recording data corresponding to the current cycle, a signal for energizing a respective heating element during an M-th subperiod, and writing means (26, 35, 36) for selectively writing a new set of recording data for the next cycle into the storage means previously holding the set of recording data corresponding to the (M-1 )-th preceding cycle, wherein said writing means is responsive to said timing means for performing the writing between the end of said first subperiod and the end of the current cycle, and the time sequential order of said first to M-th subperiods is selected such that the time interval between the end of the first subperiod and the end of the current cycle is sufficient for writing said new set of recording data. If M > 2, third means are provided for generating, depending on the respective recording data in predetermined combinations of said sets of recording data excluding the set corresponding to the (M-1 )-th preceding cycle, respective signals for energizing a respective heating element during each of the second to (M-1 )-th subperiods.

Description: The present invention relates generally to recording apparatus employing heating elements for the recording process, More particularly, the invention relates to a drive device and a drive method for driving the heating elements in such apparatus. US-A-4,364,063 discloses a thermal recording apparatus in which a line of resistive heating elements are selectively driven according to current and previous (historical) recording signals. The drive of the heating elements on not only the current but also the previous recording signals is done in order to avoid excessive heating in case the recording signals require one and the same heating element to be driven in successive recording cycles or recording lines. Thus, in a current recording cycle, less electric energy will be applied to a heating element that was energized in the preceding recording cycle, than to a heating element that was not energized in the preceding recording cycle. This is commonly referred to as "historical drive control". To accomplish such historical drive control the above mentioned document uses two or three shift registers and a latch circuit each having a number of 1-bit stages corresponding to the number of heating elements to be driven. First, the drive data for one recording line (one data for each heating element) are loaded into a first one of the shift registers and then transferred to the latch circuit. In accordance with the data in the latch circuit, a first drive pulse is applied to selected ones of the heating elements. Then the drive data are sequentially read out from the first shift register and written into the second shift register while at the same time the previous drive data are read out from the second shift register. The data of corresponding stages of the first and second shift registers are compared with each other and either are the data in the respective stage of the first shift register or that of a third shift register modified depending on the comparison result. The modified data from the first or the third shift register are then latched in the latch circuit. In accordance with the data in the latch circuit, a second drive pulse is applied to selected ones of the heating elements. This process is periodically repeated, each cycle corresponding to one recording line. As a result, in a certain cycle, each heating element to be energized in this cycle but not energized in the preceding cycle will be energized during a time corresponding to the sum of the first and second drive pulses, while heating elements to be energized in this cycle that have also been energized in the preceding cycle will be energized during the first drive pulse only. As will be appreciated from the foregoing description, in this prior art because there is only one latch circuit, it is necessary to completely replace the current drive data and the previous (historical) data each recording cycle. This limits the operating speed and increases the complexity of the control system. Another prior art disclosed in JP-A-208281/1982 seeks to eliminate this problem by providing two latch circuits and connecting the shift register to both latch circuits. In this case, one latch circuit is used to store the past drive data while the other stores the current drive data. The drawback of this method is the increased cost incurred by providing the extra latch circuit. To estimate the recording speed achievable with the prior art the following example may be considered. Assume a journal or receipt thermal printer such as it is used in a POS system, which employs a thermal head covering 4-6 cm of a paper width and having 512 dots corresponding to 512 heating elements. Then the thermal head has 512 bit shift registers. In the case of inputting the drive data to the shift register by a transfer rate of 4 Mbits/sec (4 MHz cycle), it takes 0.25 x 512 = 128 µsec. That is, in each recording cycle corresponding to one dot line, it takes 128 µsec only for the data transfer. When the printer also executes a historical drive control in accordance with US-A-4,364,063, it takes 512 µsec for exchanging the data for the historical drive control and for inputting next drive data. Furthermore, additionally considering the time for energizing the heating elements, the printing cycles become even longer, thus reducing the printing speed. When a thermal head covers A4 size paper like in a facsimile machine, the transfer time would be so great that it could no longer be ignored. EP-A-0 421 353 discloses a drive device for thermal printers employing a historical control of heating elements by storing and referencing the print data of two preceding print cycles. The print data for the current cycle and the two preceding cycles are stored in three latch circuits, respectively. When new print data are written, they are always written into the same, namely the first latch circuit after the data from the second latch circuit have been transferred to the third latch circuit and the data from the first latch circuit have been transferred to the second latch circuit. Thus, the first latch circuit holds the print data of the current cycle, the second those of the first (immediately) preceding cycle and the third those of the second preceding cycle, this assignment being fixed. Writing of new data actually requires three successive writing processes and can be performed only after the drive control in the current cycle has been completed. Since in this prior art print data are written in bit-parallel form the writing time is relatively short. In this prior art the total energizing period of the heating elements in each cycle is divided into four subperiods only three of which being used for the historical control while during a fourth subperiod (which may be the first in time sequence) a preheating pulse is applied to heating elements. The lengths of the individual subperiods may vary dependent on for instance the ambient temperature. Therefore, the object of the present invention is to provide a drive device and a drive method for a recording apparatus having recording means with a plurality of heating elements that eliminate these problems of the prior art, and, in particular, a historical drive control at a high recording speed, with good recording quality and by means of simple circuitry. This object is achieved with a drive device as claimed in claim 1, an IC as claimed in claims 10 and 11 and a method as claimed in claim 12, respectively. According to the present invention a plurality of storage means is used each having a capacity for storing drive data of a number of bits corresponding to the number of heating elements to be driven. The actual number of storage means depends on the kind of historical control, i.e. whether, in addition to the drive data of the current cycle, those of one or more preceding cycles are to be considered. If the number of preceding cycles to be considered is (M-1), the number of storage means is M. At the begin of each cycle one of the storage means holds the drive data for the current cycle while each of the remaining (M-1) storage means holds the drive data of a respective one of the (M-1) preceding cycles. Like in the prior art, the total drive or energizing period of the heating elements during each cycle is divided into at least M subperiods. A heating element which, according to the drive data for the current cycle, is to be energized will be energized for at least one of said subperiods. Whether it will also be energized during any one or more of the other (M-2) subperiods is determined by the drive data of the (M-1) preceding cycles. Each of these other subperiods is assigned to one of the preceding cycles. If a first one of these subperiods is assigned to the oldest one of the preceding cycles, the drive data of that cycle must be available during this first subperiod but are no longer required after lapse of the first subperiod. Therefore, the time after the end of the first subperiod until the end of the current cycle can be advantageously used to write the new drive data (recording data) for the next cycle into the storage means holding the oldest drive data. If the storage means are assumed to be numbered 1 through M, the number of the storage means receiving the new drive data will change in a cyclical order every period and so will of course the time rank of the drive data in the remaining storage means. In a preferred embodiment of the invention shift registers each having a serial data input and a parallel data output are used as the storage means. It should be noted that the "first" subperiod mentioned above is not necessarily the first in the time sequential order of the subperiods. Regarding the time relationship among the subperiods it is only important that the timing of the "first" subperiod be such that the time interval between the end of this first subperiod and the end of the respective cycle be long enough to allow the new data to be written. According to a first aspect the present invention provides a drive device including the number of storage means or shift registers as mentioned above and means responsive to the drive data in the storage means and to various timing control signals for correctly applying the drive data stored in the individual storage means according to their changing time rank when driving the heating elements through said subperiods, and writing means for selecting in each cycle among the shift registers that one into which the drive data for the next cycle are to be loaded and for controlling the timing of the data loading and means. According to a second aspect the present invention provides an integrated circuit including those parts of the drive device whose size or number depends on the number of heating elements that can be driven. By combining two or more such ICs any number of heating elements can be driven. According to a third aspect the invention provides a method of selectively driving a plurality of heating elements in the recording means of a recording apparatus such as the operation explained above. The total energizing period for each heating element may be divided into more than M subperiods when (M-1) preceding cycles are to be considered in the historical drive control. One or more additional subperiods may be used for preheating, for considering, in driving a particular heating element, the current and/or preceding print data of heating elements next to the particular one, etc. Also, as will be clear to those skilled in the art, the duration of the individual subperiods may be changed depending on one or more parameters like the temperature of the recording means (e.g. print head), the ambient temperature, the type of recording medium, the nature of the recording process etc. It is even possible to subdivide some or each of the subperiods to effect a pulse width or similar control in response for instance to parameters like those mentioned above. By means of this configuration and method, the current drive data and the past drive data are simultaneously stored and available when required. By sequentially and cyclically switching the data flow, it is not necessary for a control processor to determine which is the current and which is the past data. In addition, by using gate means to execute the data comparison and data referencing operations, the processor controlling the recording means of the recording apparatus can simply input the data to the serial input terminal at the energizing timing, and it is therefore sufficient to define a predetermined energizing time. Preferred embodiments of the present invention are described below with reference to the accompanying figures, in which: Fig. 1 is a basic circuit diagram of the first embodiment of a drive device according to the present invention, Fig. 2 is a timing chart used to describe an embodiment of the drive method of the invention, Fig. 3 is a basic circuit diagram of a drive control unit of a drive device according to a second embodiment of the present invention, Fig. 4 is a basic circuit diagram of the timing control unit of the second embodiment of the invention, Fig. 5 is a timing chart used to describe the drive timing of the embodiment shown in Figs. 3 and 4, and Fig. 6 is a diagram showing a cascade connection of several drive control units in combination with one timing control unit. In order to facilitate the understanding of the present invention, two embodiments will be described, a first one (Figs. 1 and 2) where the number of preceding cycles used for historical drive control is one, and a second one (Figs. 3 to 5) where this number is two. The embodiments of the invention are described as being applied to a printer as one kind of recording apparatus. It will be understood by those skilled in the art that the invention can be applied in the same way to other types of recording apparatus. The present invention can be used with all types of recording apparatus in which recording is performed by use of heating elements, including thermal printers using a thermal head, thermal transfer printers, and bubble jet-type inkjet printers that thermally produce bubbles to eject ink. In the first embodiment shown in Fig. 1, the drive device comprises a drive control unit 60 for controlling and driving N heating elements 50. Unit 60 has two shift registers 11, 12 (shift register A and shift register B) each of N 1-bit stages used as storage circuits for the print data. Each individual shift register stage 11a, 12a stores the print data for driving a heating element 50. The print data is stored in such a manner that on-state data (the heating element is to be energized) is represented by a logical 1 corresponding to a HIGH level, while off-state data (the heating element is not to be energized) is represented by a logical 0 corresponding to a LOW level. A data input/output timing control unit 40 controls the data input/output, energizing subperiods and energizing timing and is connected to input terminals 61 to 69 of the drive control unit 60. Print data SRD applied to a common data input terminal 26 and clock pulses CLK applied to a common clock input terminal 24 are supplied, via serial data input terminals 61, 63 and clock input terminals 62, 63, respectively, of drive control unit 60, to the two shift registers A and B to enable serial data input to a selected one of the shift registers. The shift registers are normally reset by applying a RESET signal to a reset terminal 27 connected to input terminal 65 of unit 60, immediately before a print job starts or at a power on state. A signal CK-A/B applied to a data input selection terminal 25 selects to which of the shift registers the new print data for the next print cycle should be transferred, and the input destination of clock input terminal 24 is selected by AND circuits 35, 36 such that the clock pulses are supplied to clock input terminal 64 and data are inputted into shift register B when data input selection terminal 25 is HIGH, while the clock pulses are supplied to clock input terminal 62 and the data are inputted to shift register A when data input selection terminal 25 is LOW. The print data are alternately inputted into the two shift registers every energizing or print cycle. The shift register used to input the first data is selected using input terminal 25, and the initial print data is inputted to the selected shift register. Because of the alternation explained above, during each cycle one of the shift registers holds the current print data while the other holds the previous print data and it is not necessary to transfer print data from one shift register to the other one. A signal EN-A/B applied to an enabling selection terminal 23 controls which of the two shift registers A and B is to be used as the one holding the current print data. When enabling selection terminal 23 is HIGH, gate circuits 31, 33 set shift register A as the current print data storage means and when it is LOW, gate circuits 31, 33 set shift register B as the current print data storage means. A first energizing subperiod control circuit is formed from gate circuits 31, 33 and a first strobe input terminal 21. The output terminals of the gate circuits 31, 33 are connected to the input terminals 66 and 68, respectively, of the control unit 60. A second energizing subperiod control circuit is formed from gate circuits 32, 34 and a second strobe input terminal 22. The output terminals of the gate circuits 32, 34 are connected to the input terminals 67 and 69, respectively, of the control unit 60. Control unit 60 includes N gate circuit blocks 70, one for each of the N heating elements. Each gate circuit block 70 is connected to a pair of corresponding stages of the shift registers A and B related to the same heating element and comprises a NAND circuit 13, AND circuits 14 to 17 and an OR circuit 18. AND circuit 17 is a first gate circuit for outputting the print data when shift register A is used for the current print data storage. AND circuit 16 is a second gate circuit for outputting the print data when shift register B is used for the current print data storage. Both of them are so constituted as to enable output of the respective print data during the pulse width of an energizing signal ST1, which is the first strobe signal input from input terminal 21, to the heating element. ST1 defines a second subperiod of the energizing time period. When the print data is 1, the heating element is energized for this second subperiod. NAND circuit 13 is a third gate circuit for outputting the NAND result of a bit comparison of the outputs of the pair of register stages in shift registers A and B driving the same heating element. AND circuit 15 is a fourth gate circuit to which the outputs of NAND circuit 13 and shift register A, which is used for the current data when shift register B is used for the historical print data, are supplied. When the compared bits are not both 1, gate circuit 15 increases the energizing time by the (first) subperiod of energizing signal ST2, which is a second strobe signal input from input terminal 22; when the compared bits are both 1 indicating a continuous on-state for the respective heating element, NAND circuit 13 outputs LOW and the energizing time is not increased. AND circuit 14 is a fifth gate circuit whose function is the same as that of the AND circuit 15 when shift register B is used for a current print data storage. Gate circuit 18 combines the output signals of AND circuits 14, 15, 16, and 17, and supplies an output to a heating element driver 19 such as a transistor. The drive method of this embodiment of the invention is described next with reference to Fig. 2, a timing chart of this drive method. Shown in Fig. 2 are a basic energizing timing signal TM, the enabling selection signal EN-A/B, the data input selection signal CK-A/B, the clock signal CLK, the serial data input signal SRD, the first strobe signal ST1, and the second strobe signal ST2. Energizing timing signal TM controls the print cycles of the thermal head, i.e., current is sequentially supplied to the thermal head at timing t1, t2, and t3 each starting a print cycle T1, T2 and T3, respectively. Immediately after the power is turned on, i.e., before timing pulse t1, all shift register data are reset. It is therefore possible to select either shift register A or B at the first timing mark. In this example, CK-A/B starts LOW, and data are therefore input to shift register A. The current cycle T1 starts at t1 after the data have been input, and ST2 becomes HIGH and the current enable signal EN-A/B is received. Because EN-A/B is HIGH at this time, the thermal head is controlled using shift register A for the current print data and referencing shift register B for the past print data. Since during this initial cycle all shift register B data are 0, print data are not continuous on-state print data. Therefore, while pulse width TW0 of ST2 is output as the first energizing subperiod, a drive current pulse of width TW0 is applied to each heating element whose corresponding bit in shift register A is 1. When ST1 then becomes HIGH, the print data from shift register A is further applied to the heating element for a second energizing subperiod equal to the pulse width TW1. In this embodiment, the total drive period for the heating elements in each cycle is divided into two subperiods TW0 and TW1. A heating element whose current print data and previous print data are both 1 is energized for only the subperiod TW1. A heating element whose current print data is 1 and whose previous print data was 0 is energized for both subperiods TW1 + TW0 wherein TW0 represents the historical drive portion. Different from the prior art, in this embodiment the historical drive portion which requires the previous print data is performed first. Thus, when ST1 becomes HIGH, only shift register A is in use, i.e., the data in shift register B at this time may be changed because the print data of the previous cycle are no longer required. Therefore, when the ST1 signal changes to HIGH that is, indicates an enabling state of outputting the present print data, next data input to shift register B begins while the heating elements are driven by means of AND circuits 17 depending on the print data in shift register A. When data input has ended and the next timing pulse t2 occurs, EN-A/B becomes LOW, thereby switching to shift register B for the current print data while using the data left unchanged in shift register A for referencing as the past print data to control the energizing time. As a result, the heating elements are optimally controlled based on the past drive history. This same energizing control is applied at print cycles T2 and T3. Thus, while the prior art require one, two or more latch circuits to store the past print data and the current print data, a preferred embodiment of the drive device according to the present invention uses shift registers and data switching circuits to efficiently store the past and current print data without needing any latch circuit. This drive device is further configured such that data input is disabled when ST2 is inputted to AND circuits 35, 36 and historical drive control is performed by means of NAND circuit 13. To this end ST2 is applied to AND gates 35 and 36 via an inverter 37 thereby to prevent clock pulses from being applied to any of the shift registers. The control sequence described above is normally controlled through a connected processor. It is therefore possible to monitor the ST2 signal at the second strobe input terminal 22 by means of the processor to prevent accidental data replacement. There is generally a significant difference between the energizing subperiod for outputting the normal (current) print data and the energizing subperiod for historical referencing, and even when there are many data bits, as with a line-type print head, there is sufficient time to input new print data into one of the shift registers during the time in which the heating elements are driven based on only the current print data stored in another one of the shift registers. As described above and shown in Fig. 1, the drive device is separated into drive control unit 60 and timing control unit 40. The drive control unit 60 comprises only those components the number or size of which corresponds to the number of heating elements to be driven, namely the shift registers 11, 12, the gate circuit blocks 70 and the drivers 19. Either can the size of the drive control unit 60 can be adjusted according to the number of heating elements in a particular recording apparatus, or it can be designed as a module for a fixed number of heating elements. In the latter case, where a particular recording apparatus has more heating elements than can be driven by a single module, plural modules can be connected in cascade. Serial data output terminals 71, 72 are provided for this purpose. In a cascade connection, output terminals 71, 72 of one module would be connected to input terminals 61, 63 of the next one while input terminals 62 and 64 to 69 of all modules would be connected in parallel. Such modular construction would allow mass production of two types of integrated circuit, one type including the components of drive control unit 60 and the other type including the common components (gate circuits 31 to 36) of the timing control unit 40. Then, because the common components are integrated on a separate IC chip, the expansion modules (control unit 60) can be mass produced at a cost reduced by the cost of the common components. The second embodiment of the present invention is described below with reference to Figs. 3 to 5. High speed thermal printers generally reference a longer print data history in order to improve print quality and extend print head life. The embodiment shown in Figs. 3 to 5 is suitable to drive the thermal head of this type of high speed thermal printer. This embodiment controls each heating element based on its print data in the current print cycle and those of the two preceding print cycles. The energizing period is divided into three subperiods TW2, TW3 and TW4 (see Fig. 5). A heating element whose print data in the current print cycle is 1 will be energized for either only TW4, TW4 + TW2 or TW4 + TW2 + TW3 depending on the drive history during the two preceding print cycles. Three shift registers are employed to hold the current print data and the two preceding ones, respectively. Like the first embodiment, the drive device of the second embodiment is made up of two basic units, a drive control unit shown in Fig. 3 (two or more of these may be cascade connected as in the first embodiment) and a data input/output timing control unit shown in Fig. 4. The shift registers 131 (C), 141 (D), and 151 (E) of the drive control unit are constructed in the same manner as the shift registers A and B of the first embodiment with respective serial data input terminals 132, 142, 152, clock input terminals 133, 143, 153, and serial data output terminals 134, 144, 154. Reset terminal 130 is common to all shift registers C-E. Gate circuit blocks 100 (only one being shown in Fig. 3) correspond to the gate circuit blocks 70 of the first embodiment. The number (total number in case of plural drive control units) of gate circuit blocks 100 equals the number of heating elements to be driven, and each gate circuit block 100 is connected to one register stage 131a, 141a, 151a in each shift register C, D, E, respectively. Each gate circuit block 100 comprises NAND circuits 121 to 124, AND circuits 111 to 119 and an OR circuit 120. NAND circuit 121 obtains the NAND of the outputs of the three register stages of shift registers C, D, E corresponding to the same heating element. NAND circuits 122, 123, 124 obtain the NAND of any two register stages in shift registers C, D, E. Signals applied to input terminals 101 to 109 control the timing and duration of drive pulses output to the heating element in accordance with the current print data and the preceding ones. AND circuits 111 to 119 output the AND result of the respective signals at these input terminals and the outputs from the NAND circuits. OR circuit 120 combines the outputs from AND circuits 111 to 119 to drive the corresponding heating element by means of a driver 110 such as a transistor. As mentioned above, if the number of 1-bit stages in each of shift registers C, D, E is N, there is a corresponding number N of gate circuit blocks 100 and head drivers 110 in the drive device, although only one each is shown in Fig. 3. Energizing subperiod control input terminals 101 to 109 are used in common for the N gate circuit blocks 100. The operation of these elements is described below. Immediately after the power is turned on, shift registers C, D, E are cleared by means of a RESET signal applied to common reset terminal 130 in the initialization process. If the print data is first input to shift register C, the shift registers are thereafter sequentially and cyclically selected synchronized to the timing for print data input in each print cycle. For example, at the next print cycle, the data in shift register C is held and the new data is inputted to shift register E. At the next print cycle, the data in shift registers C and E is held and the new data is inputted to shift register D. As this cycle is repeated, the new data will next be written to shift register C again, thereby erasing the previously stored print data. At this point shift register D will store the print data from one cycle before (1. preceding cycle) and shift register E will store the data from two cycles before (2. preceding cycle). At the cycle where the current print data are in shift register C, the data from the two previous cycles are compared by NAND circuit 121. Assuming the current print data is 1, if both previous data bits are 1, an energizing state will not result even if the corresponding input terminal 103 becomes HIGH. On the other hand, if one of the two past print data is 0, current is supplied via AND circuit 111 for the pulse width TW2 (first energizing subperiod) of the signal at input terminal 103. The data in shift register D storing the print data for the immediately preceding cycle is then referenced by NAND gate 122. If this print data is 1, NAND gate 122 outputs 0, but if it is 0, NAND gate 122 outputs 1 and current is supplied for the pulse width TW3 (second energizing subperiod) of the signal at input terminal 102. Then, based on the signal at input terminal 101, current is supplied according to the current print data stored in shift register C for the pulse width TW4 (third energizing subperiod). As described in the first embodiment above, if energization of the heating elements based on the print history is executed at the beginning of (or relatively early in) each print cycle, rewriting the past print data after that will have no adverse affect on the energizing of the current print cycle. As a result, the data stored in shift register E is rewritten by the print data for the next print cycle after referencing the data from the two cycles before the present cycle. If at any given print cycle n the new print data was input to shift register C during cycle n-1, the control process at print cycle n can be summarized as follows: reference data in shift registers C, D, E; control energizing; reference data in shift registers C, D; control energizing; rewrite data in shift register E; control energizing based only on print data from shift register C; end print cycle n; advance to print cycle n + 1; (repeat this control sequence while cyclically substituting C-D-E for each other). By thus sequentially switching through the shift registers synchronized to the print cycle, the so-called historical control referencing the past print data can be efficiently executed. The drive control unit shown in Fig. 3 can also be integrated to a single chip as can the embodiment shown in Fig. 1, and can obviously be simply applied to a line-type thermal head by increasing the number of register stages in each of the shift registers and gate circuit blocks 100 according to the number of heating elements. Fig. 4 is a circuit diagram of the timing control unit suitable for efficiently driving the drive control unit shown in Fig. 3. The connection relationships are as below: DRG1/T1...101; DRG1/T2...102; DRG1/T3...103 DRG2/T1...104; DRG2/T2...105; DRG2/T3...106 DRG3/T1...107; DRG3/T2...108; DRG3/T3...109 Referring to Fig. 4, the energizing subperiods and timing are specified by means of strobe signals ST1, ST2, ST3 applied to strobe input terminals 201, 202, 203. Enabling select terminals 204 (D0, D1) receive a 2-bit EN-C/D/E signal from which a 1-out-of-3 code is obtained by means of a decoder 211. Output signals DRG1/T1, T2, T3 to DRG3/T1, T2, T3 are obtained for application to input terminals 101 to 109 (Fig. 3) by means of AND gate circuits 221 to 229 operating on the decoder 211 output and the strobe signals ST1, ST2, ST3. DRG1/T1 is connected to terminal 101 in Fig. 3, DRG1/T2 to terminal 102, and DRG1/T3 to terminal 103. For example, when shift register C is used for the current print data, input terminals 101, 102, and 103 are used. A = 1, B = 0, C = 0 is output from decoder 211 by applying D0 = 0 and D1 = 1 of EN-C/D/E, thereby enabling data input only to AND gates 221, 222, 223. Signal DRG1/T1 is output for the ST1 input pulse width, and drive output is obtained according to the print data in shift register C. The energizing subperiod control input is similarly set from input terminals 102, 103 according to the pulse width of signals ST2 and ST3, and the output based on the past print data is applied to the heating elements. The serial data signal SRD is applied to serial data input terminal 205 and distributed as signals SD1, SD2, and SD3 to data input terminals 132, 142, 152 (Fig. 3). The clock is applied to clock input terminal 206, and the 2-bit clock select signal (CK-C/D/E) is applied to clock selection terminals 207 (D0, D1). The two bit input from clock selection terminals 207 is converted to a 1-out-of-3 code by a decoder 212. The decoder 212 output and a clock signal CLK are combined by gate circuits 231, 232, 233 to selectively obtain one of clock output signals CL1, CL2, CL3 for application to the respective shift registers C, D, E. Reset terminal 208 receives a RESET signal which is simultaneously applied to shift registers C, D, E via input terminal 130. NOR gate 230 in Fig. 4 performs a function similar to the inverter 37 in Fig. 1. NOR gate 230 disables AND gates 231, 232 and 233 if any of ST2 and ST3 is HIGH, i.e. writing of the new print data is made possible during TW4 only. Fig. 5 is a timing chart showing the drive timing of the embodiment in Fig. 3. In addition to a basic timing signal TM the signals ST1, ST2 and ST3 are shown. Each timing pulse t1 to t4 starts a new print cycle T1 to T4. As shown in this timing chart, the shift register holding the print data for the current cycle is cyclically changed in the sequence C, E, D, C every print cycle, and during each print cycle the strobe signals ST3, ST2, ST1 are output in sequence for the corresponding pulse widths TW2, TW3, TW4 at the respective output timing. As will be understood, each of TW2, TW3, TW4 constitutes a subperiod of the total energizing period (TW2 + TW3 + TW4) for the heating elements. If, in a particular print cycle, the print data in both preceding print cycles were 0, while that of the current cycle is 1, the heating element is energized for the pulse width TW2 + TW3 + TW4. If the print data of only one of the preceding print cycles was 1, the heating element is energized for TW2 + TW4; if the print data were 1 in both preceding print cycles, the heating element is energized for only pulse width TW4. According to the above description of the second embodiment, the time sequential order of the first to third energizing subperiods is: first, second, third and the new data are written in the third subperiod. This requires that the time interval of the third subperiod TW4 is sufficiently long to perform writing of the new print data for the next cycle within this time interval. Since the print data corresponding to the second preceding print cycle are not overwritten prior to the third subperiod, a time sequential order of : second, first, third subperiod or TW3, TW2, TW4 would also be possible. Even further, if the time interval of the second subperiod TW3 is sufficient for writing, the time sequential order may be: third, first, second subperiod or TW4, TW2, TW3. Of course in this latter case the new data would have to be written during the second (last) subperiod. Under the condition of this latter case, the time sequential order described in the above second embodiment would allow the new data to be written during the second (in time sequence) subperiod rather than the last one. All these alternatives are considered as being within the scope of the present claims and so are the possibilities mentioned above, namely to divide the total energizing time period (maximum current flow time) of the heating elements into more subperiods than the number of referenced preceding cycles plus 1, and/or to subdivide some or all of the subperiods. The present invention shall not be limited to the embodiments described above, and it will be obvious that the history of past print data referenced can go back three, four, or more print cycles by simply applying the above concept to increase the number of shift registers. As a result, a print head control IC and a control device for a printer using easily controllable heating elements and offering high print quality can be achieved by means of an extremely simple construction. As mentioned before, by integrating the drive control unit 60 shown in Fig. 1 or that shown in Fig. 3 into a single IC chip and mounting a plurality of such ICs on, e.g. a print head, it is possible to simplify electric patterns and easily increase the circuits corresponding to the total number of heating elements thereon. Fig. 6 is a diagram showing the location of the aforementioned ICs on a thermal head having heating elements. B1, B2, B3 and B4 are individual ICs each including a drive control unit. For example, the IC has 64 output terminals for driving 64 heating elements. Then, the four ICs together are capable of driving a total of 256 heating elements 350 on the thermal head 300. These ICs are connected in cascade by means of serial data input terminals 361, 362 and data output terminals 363, 364. Timing control unit 370 (DTC below) (including the circuitry of unit 40 in Fig. 1 or that of Fig. 4) executes control of inputting of serial data, energizing subperiods and energizing timing. In the case where the IC has an architecture of Fig. 1, it is necessary to connect 9 lines for each IC. However, from the connection in Fig. 6 it will be seen that only 9 lines have to be connected to the DTC, even though 4 ICs are used. The effect of reducing the lines is even higher when the ICs such as having an architecture of Fig. 3 wherein the number of shift registers is increased are used. When the DTC 370 is united with one drive control unit into a one chip to form IC 380 wherein lines between the DTC and drive control block are separated or constituted so as to be connected/disconnected by means of gate circuits or the like, it is possible to selectively use the DTC function among the ICs, thus dispensing with the production of another IC, reducing circuit patterns on the thermal head and increasing the reliability. Moreover, when a line head type thermal head of which the number of heating elements cover the width of A4 size paper is used in a apparatus, it is uncommon to energize all of the heating element simultaneously in consideration of the load of a power supply. In this case, the heating elements are usually controlled by being divided into several regions in the longitudinal direction. In such case, plural DTCs may be located in the respective regions each to individually control respective drive control units so that the same controlling as the above explained can be carried out in each region. Also, corresponding to this, it becomes a big advantage to unit the DTC with the drive control block into one chip IC. Although, the above explanation is made by using a thermal head, it is needles to say that the invention can be applied to the wide range of printing apparatuses using heating elements as printing elements and having a problem of the thermal accumulation. As described hereabove, data storage circuits can be efficiently used by providing plural shift registers and switching between these shift registers for the current print data and past print data according to the present invention, thereby suppressing cost increases in the head control circuit and enabling historical control referencing the past print data without sacrificing print quality. It is also possible to achieve a control method compatible with high speed system operation and providing sufficient time for data transfers without increasing the number of latch circuits as is required in the prior art. Applying the basic concept of the invention, it is also possible to easily reference three, four, or more generations of past print data, and the control method required for such operation is also extremely simple. A drive control device according to the present invention can also be flexibly adapted to line thermal heads and other thermal heads with a large number of heating elements, and the circuitry required for historical control is greatly simplified. As a result, the cost benefits of the invention are even greater with print heads having a large number of heating elements. Moreover, this invention can be applied not only to a thermal head but also to a bubble jet type ink jet head whereby the heat accumulation in the high speed printing is suppressed and stable ink ejection efficiency can take place, thus applying in the wide range.

Claims: A drive device for selectively driving, through a plurality of recording cycles, a plurality of N heating elements (50; 350) in the recording means of a recording apparatus, wherein the energizing period for each heating element in each cycle is divided into at least M successive subperiods (TW0, TW1; TW2, TW3, TW4), said device comprising: first to M-th storage means (11, 12; 131, 141, 151) for storing first to M-th sets of recording data each set including data for said plurality of heating elements, said first to M-th sets of recording data corresponding to a current cycle and the M-1 preceding cycles, timing means (21-25, 31-34) for defining said at least M subperiods, first means (13-15; 111, 114, 117, 121) for generating, depending on the respective recording data in each of the sets of recording data corresponding to the current cycle and the first to M-1-th preceding cycles, a signal for energizing a respective heating element during a first one of said subperiods, second means (17, 16; 113, 116, 119) for generating, depending on the respective recording data in the set of recording data corresponding to the current cycle, a signal for energizing a respective heating element during an M-th subperiod, if M > 2, third means (122, 123, 124, 112, 115, 118) for generating, depending on the respective recording data in predetermined combinations of said sets of recording data excluding the set corresponding to the M-1-th preceding cycle, respective signals for energizing a respective heating element during each of the second to M-1-th subperiods, and writing means (26, 35, 36; 205, 212, 231-233) for selectively writing a new set of recording data for the next cycle into the storage means previously holding the set of recording data corresponding to the M-1-th preceding cycle, wherein said writing means is responsive to said timing means for performing the writing between the end of said first subperiod and the end of the current cycle, and the time sequential order of said first to M-th subperiods is selected such that the time interval between the end of the first subperiod and the end of the current cycle is equal to or longer than the interval required for writing said new set of recording data. The drive device of claim 1, wherein each of said storage means (11, 12; 131, 141, 151) is a shift register having a number of stages corresponding to the number of heating elements (50; 350), a serial input terminal (61, 63; 132, 142, 152), parallel output terminals and a clock input terminal (62, 64; 133, 143, 153), and said writing means (26, 35, 36; 205, 212, 231-233) is means for applying a serial data stream representing said new set of recording data to the selected shift register. The drive device of claim 2, wherein said serial data stream is applied in parallel to the serial input terminals of all shift registers (11, 12; 131, 141, 151), and clock input means (35, 36; 212, 231-233) is provided for selectively applying a clock signal to the clock input terminal of the selected shift register. The drive device of claim 3, further comprising means (37; 230) for disabling said clock input means (35, 36; 212, 231-233) during said first subperiod (TW0; TW2). The drive device of any one of claims 2 to 4, wherein M equals 2, and said timing means comprises means (21, 22) for receiving first and second strobe signals (ST2, ST1), defining first and second subperiods (TW0, TW1), respectively, and timing gate means (31-34) for outputting said strobe signals to either a first group of first and second terminals (67, 66) or a second group of third and fourth terminals (69, 68), said groups of terminals being alternately used every cycle in response to a selection signal (EN-A/B) applied to said timing gate means, said second means comprises for each heating element (50) first and second gate means (17, 16) having first inputs respectively connected to the corresponding stages (11a, 12a) of the first and second shift registers (11, 12), and second inputs respectively connected to said second and fourth terminals (66, 68), said first means comprises for each heating element (50) third gate means (13) for comparing the recording data in the two corresponding stages (11a, 12a) of both shift registers (11, 12), fourth and fifth gate means (15, 14) having first inputs commonly connected to the output of said third gate means, second inputs respectively connected to the corresponding stages of the first and second shift registers, and third inputs respectively connected to said first and third terminals (67, 69), and said device further comprising for each heating element (50) sixth gate means (18) for combining the outputs of said first, second, fourth and fifth gate means. The drive device of any one of claims 2 to 4, wherein M equals 3, and said timing means comprises means (201, 202, 203) for receiving first, second and third strobe signals (ST3, ST2, ST1), defining first, second and third subperiods (TW2, TW3, TW4), respectively, timing gate means (221-229) and gate selection means (211) responsive to a selection signal (EN-C/D/E) for outputting said strobe signals to either a first group of first, second and third terminals (103, 102, 101), a second group of fourth, fifth and sixth terminals (106, 105, 104) or a third group of seventh, eighth and ninth terminals (109, 108, 107), said groups of terminals being alternately used every cycle in a cyclic order under the control of said gate selection means, said second means comprises for each heating element first, second and third gate means (113, 116, 119) having first inputs respectively connected to the corresponding stages (131a, 141a, 151a) of the first, second and third shift registers (131, 141, 151), and second inputs respectively connected to said third, sixth and ninth terminals (101, 104, 107), said first means comprises for each heating element fourth gate means (121) for comparing the recording data in the three corresponding stages (131a, 141a, 151a) of the three shift registers (131, 141, 151), and fifth, sixth and seventh gate means (111, 114, 117) having first inputs commonly connected to the output of said fourth gate means (121), second inputs respectively connected to the corresponding stages of the first through third shift registers, and third inputs respectively connected to said first, fourth and seventh terminals (103, 106, 109), said third means comprises for each heating element eighth to tenth gate means (122, 123, 124) for comparing the recording data in the two corresponding stages (131a, 141a, 151a) of each pair of the three shift registers (131, 141, 151) and eleventh to thirteenth gate means (112, 115, 118) having first inputs respectively connected to the outputs of said eighth to tenth gate means (122, 123, 124), second inputs respectively connected to the corresponding stages of the first through third shift registers, and third inputs respectively connected to said second, fifth and eighth terminals (102, 105, 108), and said device further comprising for each heating element fourteenth gate means (120) for combining the outputs of said first to third, fifth to seventh and eleventh to thirteenth gate means. The drive device of any one of the preceding claims wherein said recording means is the print head of a thermal printer. The drive device of any one of claims 1 to 6 wherein said recording means is the print head of an inkjet printer ejecting ink droplets in response to heat generated by said heating elements. The drive device of any one of claims 2 to 8, wherein said shift registers, each having a predetermined number of stages, said first, second and said third, if any, means each corresponding to said predetermined number of shift register stages are formed as a monolithcally integrated circuit having serial output terminals respectively connected to a last stage in each of said shift registers such as to allow a cascade connection of plural such integrated circuits to increase the number of heating elements that can be driven. An integrated circuit including: first to fourth timing signal input terminals (67, 66, 69, 68), two serial data input terminals (61, 63) for receiving N bits of serial data, two serial data output terminals (71, 72), two clock input terminals (62, 64), N drive output terminals (73) for connection to N heating elements, two shift registers (11, 12) each having N stages, a serial input respectively connected to said serial data input terminals, parallel outputs, a clock input respectively connected to said clock input terminals, and a serial output respectively connected to said serial data output terminals, for each drive output terminal first and second gate means (17, 16) having first inputs respectively connected to the corresponding stages (11a, 12a) of the first and second shift registers (11, 12), and second inputs respectively connected to said second and fourth timing signal input terminals (66, 68), for each drive output terminal third gate means (13) for comparing the data in the two corresponding stages (11a, 12a) of both shift registers (11, 12), fourth and fifth gate means (15, 14) having first inputs commonly connected to the output of said third gate means, second inputs respectively connected to the corresponding stages of the first and second shift registers, and third inputs respectively connected to said first and third timing signal input terminals (67, 69), and for each drive output terminal sixth gate means (18) for combining the outputs of said first, second, fourth and fifth gate means, the output of said sixth gate means (18) being connected to the corresponding drive output terminal. An integrated circuit including: first to ninth timing signal input terminals (101-109), three serial data input terminals (132, 142, 152) for receiving N bits of serial data, three serial data output terminals (134, 144, 154), three clock input terminals (133, 143, 153), N drive output terminals for connection to N heating elements, three shift registers (131, 141, 151) each having N stages, a serial input respectively connected to said serial data input terminals, parallel outputs, a clock input respectively connected to said clock input terminals, and a serial output respectively connected to said serial data output terminals, for each drive output terminal first, second and third gate means (113, 116, 119) having first inputs respectively connected to the corresponding stages (131a, 141a, 151a) of the first, second and third shift registers (131, 141, 151), and second inputs respectively connected to said third, sixth and ninth timing signal input terminals (101, 104, 107), for each drive output terminal fourth gate means (121) for comparing the data in the three corresponding stages (131a, 141a, 151a) of the three shift registers (131, 141, 151), and fifth, sixth and seventh gate means (111, 114, 117) having first inputs commonly connected to the output of said fourth gate means (121), second inputs respectively connected to the corresponding stages of the first through third shift registers, and third inputs respectively connected to said first, fourth and seventh timing signal input terminals (103, 106, 109), for each drive output terminal eighth to tenth gate means (122, 123, 124) for comparing the recording data in the two corresponding stages (131a, 141a, 151a) of each pair of the three shift registers (131, 141, 151) and eleventh to thirteenth gate means (112, 115, 118) having first inputs respectively connected to the outputs of said eighth to tenth gate means (122, 123, 124), second inputs respectively connected to the corresponding stages of the first through third shift registers, and third inputs respectively connected to said second, fifth and eighth timing signal input terminals (102, 105, 108), and for each drive output terminal fourteenth gate means (120) for combining the outputs of said first to third, fifth to seventh and eleventh to thirteenth gate means, the output of said fourteenth gate means (120) being connected to the corresponding drive output terminal. A method of selectively driving a plurality of heating elements in the recording means of a recording apparatus having first to M-th storage means for storing M sets of recording data each set including data for said plurality of heating elements, wherein said heating elements are driven through a plurality of recording cycles and said first to M-th storage means are used to store first to M-th sets of recording data corresponding to a current cycle and the M-1 preceding cycles, respectively, and each cycle includes the steps: (a) during a first energizing subperiod each heating element is driven in accordance with the respective recording data in each of the sets of recording data corresponding to the current cycle and the first to M-1-th preceding cycles, (b) if M > 2, during each of M-2 energizing subperiods other than said first one and a second one each heating element is driven in accordance with the respective recording data of predetermined combinations of said sets of recording data excluding the set of recording data corresponding to the M-1-th preceding cycle, (c) during a second energizing subperiod each heating element is driven in accordance with the respective recording data in only the set of recording data corresponding to the current cycle, and (d) a new set of recording data for the next cycle is written into the storage means previously holding the set of recording data corresponding to the M-1-th preceding cycle within a time interval between the end of said first subperiod and the end of the current cycle, wherein the time sequential order of said first to M-th subperiods is selected such that this time interval is equal to or longer than the interval required for writing said new set of recording data.

Inventors: MINOWA MASAHIRO, MINOWA, MASAHIRO
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1994
Main classification: B41J2
IPC codes: B41J2